1. Field
The embodiment(s) discussed herein are related to a magnetic recording medium in which information is recorded by controlling a magnetization direction, and a method of making the magnetic recording medium.
2. Description of the Related Art
Generally, large quantities of data are handled on a daily basis in the field of computers. One example of the devices that can read and write large quantities of data is a hard disk drive (HDD). A HDD has a built-in magnetic disk, which is a disc-shaped magnetic recording medium on which information is recorded. One example of a typical magnetic disk is a continuous medium that includes a disk composed of a nonmagnetic material and a continuous magnetic layer on the disk. In this type of magnetic disk, data is recorded by controlling the magnetization directions of crystal grains constituting magnetic domains of the magnetic layer, each magnetic domain being composed of an aggregate of a plurality of crystal grains.
One of the phenomena that obstruct long-term preservation of data recorded on a magnetic disk is a phenomenon known as heat fluctuation. Heat fluctuation is a phenomenon in which the magnetization direction of a magnetic grain becomes unstable due to influence of thermal energy from outside. Heat fluctuation becomes more and more severe as the size of the magnetic grain decreases. Since the recording density of the magnetic disks has become increasingly higher in recent years and crystal grains constituting the magnetic layer thereby has become finer, the problem of heat fluctuation has become more serious.
A perpendicular magnetic recording method for recording data by orienting the magnetization direction of each magnetic domain in a direction perpendicular to a disk surface of the magnetic disk has been known to be a method that can overcome the heat fluctuation and has thus become a mainstream technology in the field of HDDs heretofore. However, there is still an increasing demand for higher recording density. If the size of crystal grains of the magnetic disk is further reduced to achieve a higher recording density, even use of the perpendicular magnetic recording method is not enough to address the problem of heat fluctuation.
In this regard, another type of magnetic disk called a “patterned medium” described below has been proposed as one example of a magnetic disk that achieves a higher recording density and overcomes the problem of heat fluctuation (e.g., refer to Japanese Laid-Open Patent Publication Nos. 9-297918 and 10-334460).
FIGS. 1A and 1B are schematic views showing one example of a magnetic disk of a patterned medium type.
Referring to FIG. 1A, a magnetic disk 500 is a patterned medium on which data is recorded by a perpendicular magnetic recording method. Data is read from or recorded on the magnetic disk 500 with a magnetic head 551 (illustrated in FIG. 1B) mounted at a tip of a head gimbal assembly 550. FIG. 1B is an enlarged view of a region D of the magnetic disk 500 shown in FIG. 1A.
As shown in FIG. 1B, the magnetic disk (patterned medium) 500 has a plurality of small magnetic bodies (dots) 502. Each dot 502 is either a single crystal grain subjected to a forming process or an aggregate of a plurality of crystal grains subjected to a forming process, the crystal grains being strongly magnetically coupled to one another such that they magnetically behave as one crystal grain.
In this example, data is written on and read from each dot 502 with the magnetic head 551 by a perpendicular magnetic recording method. In order to do so, the crystal structure of the crystal grain is utilized. That is, the dot 502 is rendered anisotropic (crystal anisotropy) such that the magnetization direction of the dot 502 is most stable when oriented in a direction perpendicular to the surface of the magnetic disk 500. As a result, as shown in FIG. 1B, each dot 502 individually and homogeneously retains a magnetization direction M perpendicular to the surface of the magnetic disk 500. The magnetic disk 500 has such dots 502 arranged into a plurality of concentric circles on a disk 501 composed of a nonmagnetic material. Thus, a plurality of tracks 510 are formed, as shown in FIG. 1A.
In reading and writing data, the head gimbal assembly 550 moves in the direction indicated by arrow R1 in FIG. 1A to conduct tracking, and the magnetic head 551 mounted at a tip is positioned on the target track 510 from which the data is to be read or on which the data is to be written. While the magnetic head 551 is positioned as such, the magnetic disk 500 is rotated in a direction indicated by arrow R2 in FIG. 1A, and a plurality of dots 502 forming the target track 510 pass under the magnetic head 551 one after another. Data is read when the magnetic head 551 detects the magnetization direction of the dot 502 passing directly thereunder. Data is written when the magnetic head 551 applies a magnetic field to the dot 502 passing directly underneath so as to orient the magnetization direction of that dot 502 in a direction that corresponds to the direction of the applied magnetic field.
FIG. 1B shows arrangements of dots 502 of three adjacent tracks 510 (FIG. 1A) and the magnetic head 551 positioned above a center track of the three tracks 510 to read data from or write data on the target center track 510.
Unlike the continuous medium-type magnetic disk in which a minimum unit of data is written on an aggregate of a plurality of crystal grains described above, the magnetic disk 500 of a patterned medium type has a minimum unit of data recorded on the dot 502. The dot 502 is either a single crystal grain or behaves as a single crystal grain. Accordingly, while each dot 502 can be made finer to increase the recording density, the size of the crystal grain of the patterned medium-type magnetic disk 500 can be made larger than that of the continuous medium-type magnetic disk (the size of the crystal grain is a factor that affects the resistance to the heat fluctuation). In general, a patterned medium-type magnetic disk offers substantially the same recording density as that offered by a continuous medium-type magnetic disk, but with crystal grains several times to several ten times larger than the crystal grains of the continuous medium-type magnetic disk. Thus, very high resistance to heat fluctuation can be achieved.
According to the magnetic disk 500 of a patterned medium type shown in FIG. 1, the width W of the dot 502 in a direction of the radius of the magnetic disk 500 (radial direction) is equal to the width of the track 510. However, the width of the track 510 is restricted by the size of the magnetic head 551. Thus, in reducing the size of the dot 502 to achieve higher recording density, it is difficult to decrease the width W in the radial direction beyond a certain limitation. In this respect, the length L of the dot 502 in a direction extending about the center of the magnetic disk 500 (circumferential direction) is decreased to reduce the size of the dot 502. In general, a patterned medium-type magnetic disk is designed to have a bit aspect ratio (BAR) of about 2 to about 3. The BAR is a ratio of the width W of the dot 502 in the radial direction to the length L of the dot 502 in the circumferential direction.
A magnetic body having a biased shape has anisotropy dependency upon the bias of its shape (shape anisotropy) in addition to the crystal anisotropy described above. The shape anisotropy of the dot 502 of the magnetic disk 500 of a patterned medium type shown in FIGS. 1A and 1B is described below.
FIG. 2 is a diagram illustrating the shape anisotropy of the dot 502 shown in FIGS. 1A and 1B.
The dot 502 is a rectangular parallelepiped magnetic body having a length L in the circumferential direction smaller than the width W in the radial direction, as described above. The magnetization direction of a rectangular parallelepiped magnetic body tends to orient in the longitudinal direction rather than in the transverse direction. In general, the crystal anisotropy is stronger than the shape anisotropy. The magnetization direction M of the dot 502 is therefore oriented in a direction perpendicular to the surface of the magnetic disk 500, i.e., a direction along the Z axis of the coordinate system shown in FIG. 2. However, due to the shape anisotropy of the dot 502, the magnetization direction M of the dot 502 is more apt to rotate in the direction shown by arrow R4 in the Z-Y plane, i.e., in the longitudinal direction of the dot 502, than in the direction shown by arrow R3 in the Z-X plane, i.e., in the transverse direction of the dot 502, of the coordinate system.
As described above, in recording data, the magnetic head 551 is positioned, by a tracking operation, above the target track 510 on which the data is to be recorded. The magnetic head 551 applies a magnetic field to a dot 502 of the target track 510 passing thereunder to orient the magnetization direction of that dot in the direction of the applied magnetic field. The magnetization field applied from the magnetic head 551 usually has not only a component in the Z axis direction but also a component in a direction parallel to the surface of the magnetic disk (the X axis direction or the Y axis direction) so that data is recorded with a synthetic magnetic field in which these components are combined. In other words, since a slanted radial magnetic field is applied to the magnetic disk from the tip of the magnetic head 551, a magnetic field substantially in the Z-X plane and sloped with respect to the Z axis is applied to the dot 502 of the target track 510. Such a magnetic field causes the magnetization direction M to rotate in the direction shown by arrow R3, i.e., the direction in which the magnetization direction M is relatively difficult to rotate.
FIG. 3 is a schematic diagram showing the magnetic head 551 applying a magnetic field to the dot 502 passing directly thereunder.
FIG. 3 shows a dot (center dot) 502_1 of a target track (center track) 510_1 on which the magnetic head 551 records data, and two dots (side dots) 502_2 and 502_3 respectively belonging to tracks (side tracks) 510_2 and 510_3 adjacent to the center track 510_1.
As described above, in recording data, a magnetic field H1, which is substantially in the Z-X plane and slanted with respect to the Z axis, is applied from the magnetic head 551 to the center dot 502_1. As a result, the magnetization direction M1 of the center dot 502_1 is urged to rotate in a direction shown by arrow R5 in the Z-X plane.
During this operation, leakage magnetic fields H2 and H3 are applied to the vicinity of the center dot 502_1 from the magnetic head 551. The leakage magnetic field H2 sloping down to the left and substantially in the Z-Y plane is applied to the left side dot 502_2 in FIG. 3, and the leakage magnetic field H3 sloping down to the right and substantially in the Z-Y plane is applied to the right side dot 502_3 in the drawing. As a result, the magnetization direction M2 of the left side dot 502_2 is urged to rotate in a direction shown by arrow R6 in the Z-Y plane, and the magnetization direction M3 of the right side dot 502_3 is urged to rotate in a direction shown by arrow R7 in the Z-Y plane.
As described above, the rotation direction in the Z-Y plane is the direction in which the magnetization direction M of each dot 502 easily rotates due to the shape anisotropy. As a result, because of such an ease of rotation, even if the leakage magnetic fields H2 and H3 are weak, the magnetization direction M2 of the left side dot 502_2 and the magnetization direction M3 of the right side dot 502_3 sensitively react to the leakage magnetic fields H2 and H3 and may rotate.
In other words, the existing patterned medium described above may experience destruction of previously recorded data during data recording operation due to the shape anisotropy of the dots.
Although the problem of data destruction during data recording is described above by using a perpendicular magnetic recording-type patterned medium as an example, the same problem may arise with a longitudinal magnetic recording (LMR)-type patterned medium on which data is recorded by controlling the magnetization directions in the in-plane directions of the recording disk. Moreover, although oversensitive response, caused by shape anisotropy, of the magnetization directions of the dots in the side tracks toward the leakage magnetic fields is described above as an example of the cause of destruction of previously recorded data, the cause of the magnetization rotation under the leakage magnetic fields is not limited to this. For example, even when the dot has no shape anisotropy, the same problem may occur when the track width is excessively narrowed to increase the data recording density and the magnetization of the dots of the side tracks are thus more sensitive to leakage magnetic fields.